Questions below are a synopsis of what we've
been asked when we've presented the research that has led to this book
at various international meetings (American Geophysical Union; American
Meteorological Society; European Geosciences Union), national
laboratories (NASA Goddard Space Flight Center, National Center for
Atmospheric Research), and universities (Columbia University, Colorado
State University, Harvard University, Johns Hopkins University, Toronto
University, University of Maryland).

What is your agenda?

This is a fairly frequent question and,
since it is fairly loaded, we have chosen to place our answer to
this question first.

Our agenda is simple: the truth,
the whole truth, and nothing but the truth.

We're confident however this
is how nearly all scientists would reply.

For what it is worth, let us add:

we
did not set out to show the CMIP5 (Climate Model Intercomparison
Project, Phase 5) GCMs (atmospheric, oceanic general circulation
models) warm too quickly.
Rather, we stumbled across this result upon a fairly straight
forward analysis of the climate record. We then devoted a major
internal effort to first assessing and then shoring up this finding, by considering a full range of possible uncertainties in both
our empirical evaluation of the human influence on global
warming as well as our extraction of this signal from the CMIP5 climate models

we
will derive no financial benefit from this book: the entire
content is available free of charge to anyone in the world, via
Open Access. In return for arranging Open Access, we forfeited
any royalties from the sale of the book. We hope the book sells
many copies. If so,
this shall financially benefit the book publisher and not the five co-authors

the
salaries of the co-authors of this book have been supported by
the selection of a peer reviewed proposal submitted to the NASA
Climate Indicators and Data Products for Future National Climate
Assessment (INCA) program as well as by the State of Maryland
(i.e., two of us teach full time; the other three have taught part time). The words in the book reflect only our views:
we do not speak for NASA, the State of Maryland, our University,
or anyone else

while
we have spoken on this topic at various international meetings,
national laboratories, and universities, we have never received
an honorarium. On rare occasion, a
host has paid our travel cost. Usually our talks are given
when a speaker happens to be in town for some other event, such
as a scientific workshop

Why should we believe your climate model
projections when so many others, such as those who worked
on IPCC (2013), disagree?

This question, also
fairly frequent, usually comes with a heap of scorn, a dose of ridicule, plus a
topping of "you must not have read the IPCC report".

We are intimately familiar with
the fourteen Chapter Physical Science Basis, 2013 report issued by the
Intergovernmental Panel on Climate Change.

We teach from the report: the
five co-authors have been involved with three courses at the
University of Maryland (Introduction
to Weather and Climate; The Science, Governance, and Economics of
Climate Change; Atmospheric Chemistry and Climate) that utilize
material from various IPCC reports.

We invite those quick to assert
that our findings disagree with the Physical Science Basis report of
IPCC (2013) to have a careful look at Figures
11.25 and
TS.14 of
this document. These figures show a trapezoid described by the
authors of Chapter 11 and the Technical Summary as the
indicative likely range of global mean surface temperature (GMST)
during the period 2016–2035, based the IPCC author teams' expert
assessment of the GCM output.

The IPCC trapezoid, shown in
green on the figure above, is featured
prominently in our book. The projections of GMST found using
our Empirical Model of Global Climate are in remarkably good
agreement with the IPCC projection of how GMST will rise
over the next several decades.

To suggest a fundamental
difference between the EM-GC projections of global warming and the
expert judgement of IPCC indicates a lack of
familiarity with both Chapter 11 and the Technical Summary of IPCC (2013).

If the GCMs warm too quickly, what is wrong with
these models?

It is hard for some to fathom that so many of the large climate models could be in
error. Generally, we are asked this question by colleagues
who don't work directly with the development of these models.

As explained in Section 2.3 of
our book, the most likely reason the CMIP5 GCMs warm too quickly is
that climate feedback within these models is too large.
Climate feedback refers to responses within Earth's climate system
that either amplify ("positive feedback") or dampen ("negative
feedback") the initial perturbation, which in this case is warming
driven by an increase in the radiative forcing of climate (RF) due
to human release of greenhouse gases (GHGs).

As outlined in our book, cloud
feedback tends to be positive in nearly all GCMs. It has proven to
be quite challenging to define cloud feedback from observations.
There are major international efforts underway to improve both the
empirical determination of cloud feedback (one of the book
co-authors is presently serving on a
US National Academy Panel focused on this and many other
observational needs) as well as the
evaluation and representation of cloud feedback within climate
models. Finally, we note that two of
the 42 CMIP5 GCMs (INM-CM4, from the Institute for Numerical
Mathematics, Russian Academy of Sciences and MRI-CGCM3 from the
Meteorological Research Institute, Japan) we have examined exhibit a
value of attributable, anthropogenic warming rate (AAWR) over the
past three decades that is in very close quantitative agreement with
the value of AAWR inferred from the climate record (see Table 2.3 of
our book). Our empirical model of global climate does exhibit
characteristics similar to some of the established GCMs. The reason
for the better agreement with the simulations of climate submitted
to the CMIP5 archive by these groups is an area of active research.

The RCP 4.5 scenario has
atmospheric levels of CH4 being nearly unchanged during the course of this century. How does this complicate your
identification of RCP 4.5 as the 2°C
pathway?

We are almost always asked about methane
(CH4).

The future trajectory of
atmospheric CH4 is vitally important.

Our climate modeling effort
identifies RCP 4.5 as the 2°C pathway and this is indeed complicated
by the fact that the atmospheric mixing ratio of CH4 at
the end of this century in RCP 4.5 is 1.6 parts per million (ppm),
lower than the present value of about 1.84 ppm.

This complication is the focus of
Section 4.4.2 of our book. The production of electricity by
the combustion of natural gas (which is mainly CH4)
yields about 70% more power, per atmospheric CO2 molecule
released, than the combustion of coal. Many countries are
transitioning from coal to natural gas, driven by both economic
factors (i.e., new abundant supplies of natural gas, such as the
source supplied by fracking in the US) and air quality concerns
(surface ozone and particulate loading, the pollutants that drive
air quality, fare much better when the use of coal is reduced).

There are two difficulties with
this transition. First, if CH4 leaks to the
atmosphere, the transition to natural gas can impose a climate
penalty, rather than provide a climate benefit, since CH4
is a more potent GHG than CO2. The image below,
based on Figure 4.12 from our book, quantifies the probability of
achieving the goal (1.5°C) or upper limit (2.0°C) of the Paris
Climate Agreement as a function of the future evolution of CH4:

The second and perhaps more acute
problem with the transition from coal to natural gas is related to the
question of whether or not CH4 will truly be a transition
fuel. As shown on the summary page
and detailed in Chapter 4, in order to place global emissions of
GHGs on the RCP 4.5 pathway, half of the world's energy must be
produced by sources that do not release GHGs by year 2060.
Electric Generation Units (EGUs) commissioned in the next few years
will likely be designed with a lifetime that will extend past 2050.
Therefore, CH4 is vitally important for assessing how to
achieve the goals of the Paris climate agreement.

Wasn't there a global warming hiatus?

This question is so
"2014"! But we include it here, nonetheless. We had
often been asked this question, of course before the recent
ENSO-driven rise in global mean surface temperature.

The so-called Global Warming
Hiatus is the subject of Section 2.4 of our book. There had
been considerable attention devoted to an apparent cessation of
global warming, for analyses of temperature trends from the start of 1998 to the
end of 2012.

To make a long story short, the
skeptics of global warming seized upon this oddity, which was driven by a
strong El Niño Southern Oscillation (ENSO) climate event that began
in September 1997. This reorganization of the tropical Pacific Ocean adrove
a rapid increase in global mean surface temperature in early 1998,
the start of the hand-picked time interval for analysis of the
hiatus.
Climate scientists "took the bait" and there has
been an extensive amount of scholarly activity devoted to this
topic, much of which is discussed in our book.

Our modeling efforts are in line
with the interpretation first
published by
Karl
et al. (2015)that there was never a true hiatus in the rise of
global mean surface temperature. Rather, over the 1998 to 2012 time
period, the rise in temperature was somewhat slower than had
occurred previously (and has occurred since) due to two factors:
the tendency of the climate system to be in a more La Niña-like
state during the later half of this time interval, coupled with a
smaller than normal rise of total solar irradiance during the period
of peak activity of the most recent solar cycle (number 24 for those
keep score). Our Empirical Model of Global
Climate provides remarkably good simulation of the rise in
temperature over the time period of the so-called hiatus:

Your forecasts suggest global warming could stay
beneath the Paris target of 1.5°C, but the past two years have seen
global average temperature almost at this level. Doesn't that mean your
hypothesis can already be shown to be wrong?

We've gotten a
similar question at a few recent presentations and we anticipate this
will be a primary source of criticism on our contention that RCP
4.5 is the 2°C pathway.

To quote the great Yogi Berra,
"it is deja-vu all over again".

First we had the called Global
Warming Hiatus, which was driven by the ENSO event of 1997-98.
Now we have a rapid intensification of global warming that is
clearly linked to the ENSO event that begin around May 2015 (see
https://svs.gsfc.nasa.gov/30645 for a nice animation of sea
surface temperature), which has led to many news stories focused on
how close we are to reaching the Paris Climate Agreement goal of
1.5°C global warming.

Our Empirical Model of Global
Climate accounts for the effect of ENSO. We considered
observations of global mean surface temperature from three of the
leading data centers, until the end of 2015, in all of the published
figures.

Past ENSO events show that global
temperature will decline back to ENSO-neutral conditions within a
year of the ENSO event. Such a decline is readily apparent in the latest available
data from CRU at the time of book release (i.e., Jan 2017):

Our global warming forecasts
(red lines marked RCP 4.5 & RCP 8.5 above) assume neutral conditions for ENSO, major volcanoes, solar
irradiance, as well as the Atlantic Meridional Overturning
Circulation and the Pacific Decadal Oscillation. As such, since
the end of 2015 was marked by anomalous warming driven by the
ENSO event that began around May 2015, there is a bit of a "data
shock" in some of the figures (i.e., Fig 2.19 and 2.20)
shown in the book. It just so happened we went
to press in the middle of an extreme ENSO event. Rather
than try to model the precise timing of the recovery from ENSO,
which as illustrated above can take many months, we instead
focused on the use of radiative forcing of climate due to
anthropogenic factors to drive our forecasts of global warming.

Would you bet the world that your model
is correct?

We were asked this question when
presenting at a session advocating for the divestment of Universities from investments in fossil fuels, at the Climate Action 2016
meeting.

Finally, an easy question to
answer!

No, we would not bet the world
our model is correct. In fact, we would not bet the world the
sun will come up tomorrow.

After all, it is the world!!!
For all we know, someone has placed a device at the L1 point that
will block sunlight from reaching the Earth tomorrow.

In all seriousness ... we are not
suggesting that the climate projections found using our EM-GC
replace those of the CMIP5 GCMs. A huge amount of
research effort has gone into the CMIP5 GCMs. These models
forecast a myriad of policy relevant climate variables such as
precipitation, sea level rise, the state of the cryosphere, and
conditions conducive to severe weather that our simple model does
not address. Nonetheless, global mean surface temperature, which our
model has been designed to forecast, has emerged as an important
policy relevant metric upon which the Paris Climate Agreement will
be assessed.

As stated in our book (Preface
and Chapter 2):

We urge that judgement of our EM-GC projections
of global warming be based on whether other research groups are
able to reproduce these findings, using similar types of analyses. Given
these caveats, our forecasts of global warming suggest that GHG emissions of RCP 4.5 constitute a reasonable guideline for
attempting to achieve both the Paris target (1.5°C) and
upper limit (2.0°C) for global warming, relative to the
pre-industrial era. We urge decision makers to seek their own
independent assessments of the veracity of all global warming
projections being used to inform policy.